12/14/2016. Errors. Debugging and Error Handling. Run-Time Errors. Debugging in C# Debugging in C# (continued)

Transcription

1 Debugging and Error Handling Debugging methods available in the ID Error-handling techniques available in C# Errors Visual Studio IDE reports errors as soon as it is able to detect a problem Error message Syntax errors does not always Language rule violation state the correct problem Quick info Syntax error extraneous semicolon Run-Time Errors Just because your program reports no syntax errors does not necessarily mean it is running correctly One form of run-time error is a logic error Program runs, but produces incorrect results May be off-by-one in a loop Sometime users enter incorrect values Finding the problem can be challenging Debugging in C# Desk check Many IDEs have Debuggers Debuggers let you observer the run-time behavior You can break or halt execution You can step through the application You can evaluate variables You can set breakpoints Debug menu offers debugging options Debugging in C# (continued) Debugging in C# (continued) Select Start Debugging and number of options to run your program doubles Debug menu options Debug menu options during debugging mode 1

2 Breakpoints Markers placed in an application, indicating the program should halt execution when it reaches that point Break mode Examine expressions Check intermediate results Use Debug menu to set Breakpoint F9 (shortcut) Toggles Breakpoints (continued) Red glyph placed on the breakpoint line Breakpoint set Break Mode Break Mode (continued) In Break mode, Debugger displays Locals window All variables and their values are shown Locals window at the breakpoint Breakpoint location Debugging in C# Continue Takes the program out of break mode and restores it to a run-time mode If more than one breakpoint set, Continue causes the program to execute from the halted line until it reaches the next breakpoint Stepping through code Execute code line by line and see the execution path Examine variable and expression values as they change Stepping Through Code Step Into (F11) Program halts at the first line of code inside the called method Step Over (F10) Executes the entire method called before it halts Step Out (Shift+F11) Causes the rest of the program statements in the method to be executed and then control returns to the method that made the call 2

3 Watches Can set Watch windows during debugging sessions Watch window lets you type in one or more variables or expressions to observe while the program is running Watch window differs from Locals window, which shows all variables currently in scope Quick Watch option on Debug menu lets you type a single variable or expression Watches (continued) QuickWatch window Bugs, Errors, and Exceptions Bugs, Errors, and Exceptions (continued) Bugs differ from exceptions Bugs, also called "programmer mistakes," should be caught and fixed before application released Errors can be created because of user actions Example Entering wrong type of data produces unhandled exception when ParseInt( ) called Details button in Visual Studio lists a stack trace of methods with the method that raised the exception listed first Stack trace Figure Unhandled exception raised by incorrect input string Exception-Handling Techniques If event creates a problem frequently, best to use conditional expressions to catch and fix problem Execution is slowed down when CLR has to halt a method and find an appropriate event handler Exception-handling techniques are for serious errors that occur infrequently Exceptions classes integrated within the FCL Used with the try catch finally program constructs What is an exception error? An exception is a problem that arises during the execution of a program. A C# exception is a response to an exceptional situation that arises while a program is running, such as an attempt to divide by zero. 18 3

4 What is an exception error? Exception handling Exceptions provide a way to transfer control from one part of a program to another. C# exception handling is built upon four keywords: try: A try block identifies a block of code for which particular exceptions will be activated. It's followed by one or more catch blocks. catch: A program catches an exception with an exception handler at the place in a program where you want to handle the problem. The catch keyword indicates the catching of an exception Exception handling C# exception handling is built upon four keywords: finally: The finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For example, if you open a file, it must be closed whether an exception is raised or not. throw: A program throws an exception when a problem shows up. This is done using a throw keyword. DEBUGGING IN VS AND VCE Both VS and VCE(express) allow you to build applications in two configurations: Debug (the default) and Release. 21 DEBUGGING IN VS AND VCE In debug configuration and execute it in debug mode, more is going on than the execution of your code. Debug builds maintain symbolic information about your application, so that the IDE knows exactly what is happening as each line of code is executed In release configuration, application code is optimized. However, release builds also run faster; and when you have finished developing an application, you will typically supply users with release builds because they won t require the symbolic information thatdebug builds include Debug Menu and Toolbar Breakpoints 4

5 Breakpoints Breakpoints Debugging in Break Mode Breakpoints Toggle Breakpoints On/Off by clicking in Editor's gray left margin indicator Monitoring Variable Content Monitoring variable content is just one example of how VS and VCE help you a great deal by simplifying things. The easiest way to check the value of a variable is to hover the mouse over its name in the source code while in break mode. A yellow tooltip showing information about the variable appears, including the variable s current value. Viewing Current Values During Program Execution Place mouse pointer over variable or property to view current value Monitoring Variable Content Autos (VS only): Variables in use in the current and previous statements (Ctrl+D, A) Locals: All variables in scope (Ctrl+D, L) Watch N: Customizable variable and expression display (where N is 1 to 4, found on Debug WindowsWatch) Immediate and CommandWindows The Command (VS only) and Immediate windows (found on the DebugWindows menu) enable you to execute commands while an application is running. The Command window enables you to perform VS operations manually (such as menu and toolbar operations). The Immediate window enables youto execute additional code besides the source code lines being executed, and to evaluate expressions. 5

6 Immediate and CommandWindows Debugging and Error Handling Error-handling available in C# techniques UNDERSTANDING EXCEPTIONS STEP BY STEP 3_1 An exception occurs when a program encounters any unexpected problems. Your program should be able to handle these exceptional situations and, if possible, gracefully recover from them. This is called exception handling. STEP BY STEP 3_1 UNDERSTANDING EXCEPTIONS The FCL provides two categories of exceptions ApplicationException Represents exceptions thrown by the applications SystemException Represents exceptions thrown by the CLR 6

7 try block A try block contains code that requires common cleanup or exception-recovery operations. The cleanup code should be put in a single finally block. The exception recovery code should be put in one or more catch blocks. Create one catch block for each kind of type you want to handle. A try block must have at least one catch or finally block. catch block A catch block contains code to execute in response to an exception. If the code in a try block doesn t cause an exception to be thrown, the CLR will never execute the code in any of its catch blocks. You may or may not specify a catch type in parantheses after catch : The catch type must be of type System.Exception or a type that derived from System.Exception If there is no catch type specified, that catch block handles any exception. This is equivalent to having a catch block that specifies System.Exception as a catch type. CLR searches for a matching catch type from top to bottom. If CLR cannot find any catch type that matches the exception, CLR continues searching up the callstack to find a catch type. catch block Once the catch block that matches the exception is found, you have 3 choices: 1. Re-throw the same exception, notifying the higher-up call stack ofthe exception 2. Throw a different exception, giving richer exception information to code higher-up in the call stack 3. Let the code continue from the bottom of the catch block In choices 1-2, an exception is thrown and code starts looking for a catch block whose type matches the exception thrown In choice 3, the finally block is executed You can also specify a variable name like catch(exception e) to access information specific to the exception. finally block The CLR does not completely eliminate memory leaks. Why? Even though GC does automatic memory clean-up, it only cleans up if there are no references kept on the object. Even then there may be a delay until the memory is required. Thus, memory leaks can occur if programmers inadvertently keep references to unwanted objects. C# provides the finally block, which is guaranteed to execute regardless of whether an exception occurs. If the try block executes without throwing, the finally block executes. If the try block throws an exception, the finally block still executes regardless of whether the exception is caught. This makes the finally block ideal to release resources from the corresponding try block. finally block Local variables in a try block cannot be accessed in the corresponding finally block, so variables that must be accessed in both should be declared before the try block. Placing the finally block before a catch block is a syntax error. A try block does not require a finally block, sometimes no clean-up is needed. A try block can have no more than one finally block. Avoid putting code that might throw in a finally block. Exception handling will still work but the CLR will not keep the information about the first exception just thrown in the corresponding try block. Try Block - General Form The try Block try statements that may cause error catch [ExceptionType VariableName ] statements for action when an exception occurs 7

8 HANDLING EXCEPTIONS The catch Block HANDLING EXCEPTIONS DivideByZeroException ArithmeticException FormatException OverflowException HANDLING EXCEPTIONS HANDLING EXCEPTIONS The throw Statement The throw Statement The throw Statement 8

9 HANDLING EXCEPTIONS The finally Block: contains code that always executes, whether or not any exception occurs. Example 1 1. class Program 2. public static void division(int num1, int num2) float result=0.0f; 5. try result = num1 / num2; catch (DivideByZeroException e) Console.WriteLine("Exception Error!! \n divid by zero!!"); 12. // Console.WriteLine("Exception caught: 0", e); finally Console.WriteLine("Result: 0 ", result); static void Main(string[] args) division(10,0); 22. Console.ReadLine(); Example 2 Practice 1. catch (DivideByZeroException ex2) 2. Try ex1.message 3. // catch (FormatException ex1) // 8. Try Exception only Instead of DivideByZeroException Or FormatException 51 Use IndexOutOfRangeException & FormatException & Exception 52 VALIDATING USER INPUT The Validating Event Field-Level Validation 1. Enter (Occurs when a control is entered.) 2. GotFocus (Occurs when a control receives focus.) 3. Leave (Occurs when focus leaves a control.) 4. Validating (Occurs when a control is validating.) 5. Validated (Occurs when a control is finished validating.) 6. LostFocus (Occurs when a control looses focus.) Inside the Validating event, you can write code to do the following: Programmatically correct any errors or omissions made by the user. Show error messages and alerts to the user so that the user can fix the problem Use the Focus() method of the control to transfer the focus back to the field. Set the Cancel property of CancelEventArgs to true. This cancels the Validating event, leaving the focus in the control. 9

10 VALIDATING USER INPUT 1. KeyDown 2. KeyPress 3. KeyUp The KeyPress event happens after the KeyDown event but before the KeyUp event KeyPress event match keys include any alphabetic and numeric characters (alphanumeric a z, A Z, and 0 9), not raise this event include Ctrl, Alt, and the function keys VALIDATING USER INPUT EXAM private void textboxage_keypress(object sender, KeyPressEventArgs e) if ((e.keychar < 48 e.keychar > 57) && e.keychar!= 8) e.handled = true; private void textbox1_keyup(object sender, System.Windows.Forms.KeyEventArgs e) if (e.alt == true) MessageBox.Show("The ALT key is still down"); Methods Math Class Methods The Math class Allows the user to perform common math calculations Using methods ClassName.MethodName( arument2, ) Constants Math.PI = Math.E = argument1, Method Definitions Writing a custom method Header ReturnType Properties Name( Param1, Param2, ) Body Contains the code of what the method does Contains the return value if necessary For uses call elsewhere in program Pass parameters if needed All methods must be defined inside of a class Method Definitions public void MethodName ( ) // Contains the code of what the method does public ReturnType methodname(param1, Param2, ) //Contains the code of what the method does //Contains the return value 10

11 User-defined method Maximum User-defined method Maximum public double Maximum( double x, double y, double z ) double maximumvalue = x; if ( y > maximumvalue ) maximumvalue = y; if ( z > maximumvalue ) maximumvalue = z; return maximumvalue; // end method Maximum public void DetermineMaximum() Console.WriteLine( "Enter three floating-point values,\n" + " pressing 'Enter' after each one: " ); double number1 = Convert.ToDouble( Console.ReadLine() ); double number2 = Convert.ToDouble( Console.ReadLine() ); double number3 = Convert.ToDouble( Console.ReadLine() ); double result = Maximum( number1, number2, number3 ); Console.WriteLine( "Maximum is: " + result ); Argument Promotion Argument Promotion Implicit Conversion Object is converted to a needed type implicitly Only done if complier knows no data will be lost Explicit Conversion Object is manually converted Required if there could be a loss of data Widening Make an object that of a derived class and more complex Narrowing Make an object that of a base class and cause some data loss Type bool byte sbyte char decimal double float int uint long ulong object short ushort Can be Converted to Type(s) object decimal, double, float, int, uint, long, ulong, object, short or ushort decimal, double, float, int, long, object or short decimal, double, float, int, uint, long, ulong, object or ushort object object double or object decimal, double, float, long or object decimal, double, float, long, ulong, or object decimal, double, float or object decimal, double, float or object None decimal, double, float, int, long or object decimal, double, float, int, uint, long, ulong or object Passing Arguments: Call-By- Value vs. Call-By-Reference Passing by value Send a method a copy of the object When returned are always returned by value Set by value by default Passing by reference Send a method the actual reference point Causes the variable to be changed throughout the program When returned are always returned by reference The ref keyword specifies by reference The out keyword means a called method will initialize it Reference and value parameters void Square( int x ) x = x * x; int z=5; Value of z after Square: 5 Square ( z ); void SquareRef( ref int x int ) z=5; Value of z after SquareRef: 25 x = x * x; SquareRef (ref z ); void SquareOut( out int x ) x=5; x = x * x; int z; Value of z after SquareOut: 25 SquareOut (out z ); 11

12 Random Number Generation Class Random Within namespace System randomobject.next() Returns a number from 0 to Int32.MaxValue Int32.MaxValue = 2,147,483,647 randomobject.next ( x ) Returns a value from 0 up to but not including x randomobject.next ( x, y ) Returns a number between x and up to but not including y Class Random_ Example Random rand = new Random(); int value; value = rand.next(); // [0; 2,147,483,647] value = rand.next( 6 ); // [0; 5] value = rand.next( 1, 7 ); // [1,6] 12